For machining materials with lasers, the machining result is very critically dependent upon the energy distribution at the focus. This applies especially to laser cutting. Here, lasers are preferably utilized which radiate in a low mode and therefore supply an approximately diffraction limited focus distribution. An adaptation of the focus distribution to the requirements (for example, determined by the type of material and the material thickness) can, in this case, be obtained practically only by means of the effective aperture and therefore the focal length of the focussing optics. For this reason, different focal lengths are used depending upon the application. When the requirements change frequently, then the need for focussing optics having a changeable aperture increases in order to reduce the change-over time.
Such systems have long been utilized when using lens optics. Here, complete pancratic focussing units or pancratic expansion or contraction systems in combination with fixed focussing optics are utilized.
Lens systems are only of use to a limited extent when utilizing lasers of high capacity. For this reason, the lens systems are increasingly replaced with mirror optics which can be loaded to a significantly greater extent. However, only embodiments having a fixed aperture are known.